This invention relates to improvements in decoders for many-carrier signals, in particular in DVB-T receivers.
The invention is described in the context of the reception of so-called DVB-T signals, that is signals in accordance with the Digital Video Broadcasting standard for Terrestrial television, as defined in European Telecommunication Standard ETS 300 744. The invention in its various aspects is not however necessarily limited to use with such signals, but may be usable with other coded many-carrier transmission systems, particularly though not exclusively those using orthogonal frequency-division multiplexing (OFDM).
The DVB-T standard specifies the use of two types of interleaving in the forward error corrector in the transmitter. These comprise an outer convolutional interleaver and an inner interleaver. The effect of these interleavers is to spread the data in time, so that a short error burst does not affect bits which are adjacent in the original signal, but rather affects bits which are spread out over a period of time. This reduces the instantaneous error rate to a level where the errors can be corrected by the error correction circuitry in the receiver.
This works well for most types of transmission error. However, we have found that it can fail when there is impulsive interference with bursts of interference which have a duration of greater than, say, 10 μs. What happens then is that the interleaving spreads the errors in time but the error rate is still too high for the error correction circuitry, and thus the errors persist for a longer duration than they would have done if there was no interleaving at all.
With a many-carrier system such as coded OFDM (COFDM), the interference is spread over many samples by the time domain/frequency domain transformation effected e.g. by a fast Fourier transform (FFT) circuit. The adverse effect just described is therefore more likely to arise than with a conventional single-carrier system.
The present invention is directed to various ways in which this adverse effect of impulsive interference can be ameliorated.
It would be possible to achieve some improvement by increasing the depth of the outer interleaving, so that instead of being spread over 12–15 packets the errors were spread over say 105 packets, A larger memory is now required for the outer deinterleaver in the receiver and the system could still fail unless the interval between bursts of impulses were fairly long.
Another possibility would be to increase the redundancy in the Reed-Solomon code in the outer coder of the DVB-T standard. Increasing the redundancy will, however, correspondingly decrease the data capacity, and would upset the convenient numerical relationship adopted in the standard which leads to a whole number of packets in a super-frame.
Reference may be made to U.S. Pat. No. 5,687,164 and European Patent Applications EP-A-0 827 298, 0 827 299, and 0 827 300 which describe the use of a Viterbi decoder to decode a QAM signal. International Patent Application WO97/13378 describes obtaining channel state information from QPSK modulated symbols. U.S. Pat. No. 5,636,253 describes selecting for each symbol the nearest of the QAM points and calculating the difference from it, and multiplying it with a weighting coefficient. International Patent Application WO92/22162 describes a QAM system in which the constellation is modified in accordance with channel quality. U.S. Pat. No. 5,134,635 describes a convolutional decoder which assigns bit metrics to at least one bit of a symbol, and uses soft-decision Viterbi decoding with channel state information.